Unknowns in Stem Cell/Cloning Research

It is uncertain that human embryonic stem cells in vitro
can give rise to all the different cell types of the
adult body.

It is unknown if stem cells cultured in vitro (apart
from the embryo) will function as the cells do when they
are part of the developing embryo.

It is unknown if embryonic stem cell lines will
continue to proliferate indefinitely or will undergo
genetic Permanent structural alterations in DNA, consisting of either substitutions, insertions or deletions of nucleotide bases.mutations and fail to be useful.

Human embryonic stem cells
have been studied only recently, so their capabilities are, as of yet,
unknown. In theory, the embryonic stem cells are able to form every cell
type (which is what they do in the embryo). However, the conditions in
culture might not be able to recreate the conditions that give rise to
many tissues in the intact embryo. In addition to these unknowns, it is
uncertain that the cultured stem cells will function the same as cells
that have been developed within the embryo. For example, in a recent
study, insulin-producing cells derived from murine embryonic stem cells
failed to produce the insulin when transplanted into mice, but only
formed tumors.1 In addition, it is
uncertain that these lines will continue to proliferate indefinitely
without undergoing genetic Permanent structural alterations in DNA, consisting of either substitutions, insertions or deletions of nucleotide bases.mutations that render them useless. According
to the President's Council on Bioethics:

"It is not yet known whether any
preparation of human ES cells (generally believed to be much
longer-lived than adult stem cells) will continue to grow
'indefinitely,' without undergoing genetic changes."2